Dopamine turnover increases in asymptomatic LRRK2 mutations carriers

Multi-tracer PET correlation analysis reveals disease-specific patterns in Parkinson's disease and asymptomatic LRRK2 pathogenic variant carriers compared to healthy controls

Several genetic pathogenic variants increase the risk of Parkinson's disease (PD) with pathogenic variants in the leucine-rich repeat kinase 2 (LRRK2) gene being among the most common. A joint pattern analysis based on multi-set canonical correlation analysis (MCCA) was utilized to extract PD and LRRK2 pathogenic variant-specific spatial patterns in relation to healthy controls (HCs) from multi-tracer Positron Emission Tomography (PET) data. Spatial patterns were extracted for individual subject cohorts, as well as for pooled subject cohorts, to explore whether complementary spatial patterns of dopaminergic denervation are different in the asymptomatic and symptomatic stages of PD. The MCCA results are also compared to the traditional univariate analysis, which serves as a reference. We identified PD-induced spatial distribution alterations common to DAT and VMAT2 in both asymptomatic LRRK2 pathogenic variant carriers and PD subjects. The inclusion of HCs in the analysis demonstrated that the dominant common PD-induced pattern is related to an overall dopaminergic terminal density denervation, followed by asymmetry and rostro-caudal gradient with deficits in the less affected side still being the best marker of disease progression. The analysis was able to capture a trend towards PD-related patterns in the LRRK2 pathogenic variant carrier cohort with increasing age in line with the known increased risk of this patient cohort to develop PD as they age. The advantage of this method thus resides in its ability to identify not only regional differences in tracer binding between groups, but also common disease-related alterations in the spatial distribution patterns of tracer binding, thus potentially capturing more complex aspects of disease induced alterations.

Imaging Biomarkers in Prodromal and Earliest Phases of Parkinson's Disease

Assessing imaging biomarker in the prodromal and early phases of Parkinson's disease (PD) is of great importance to ensure an early and safe diagnosis. In the last decades, imaging modalities advanced and are now able to assess many different aspects of neurodegeneration in PD. MRI sequences can measure iron content or neuromelanin. Apart from SPECT imaging with Ioflupane, more specific PET tracers to assess degeneration of the dopaminergic system are available. Furthermore, metabolic PET patterns can be used to anticipate a phenoconversion from prodromal PD to manifest PD. In this regard, it is worth mentioning that PET imaging of inflammation will gain significance. Molecular imaging of neurotransmitters like serotonin, noradrenaline and acetylcholine shed more light on non-motor symptoms. Outside of the brain, molecular imaging of the heart and gut is used to measure PD-related degeneration of the autonomous nervous system. Moreover, optical coherence tomography can noninvasively detect degeneration of retinal fibers as a potential biomarker in PD. In this review, we describe these state-of-the-art imaging modalities in early and prodromal PD and point out in how far these techniques can and will be used in the future to pave the way towards a biomarker-based staging of PD.

Inhibition of LRRK2 kinase activity rescues deficits in striatal dopamine physiology in VPS35 p.D620N knock-in mice

Dysregulation of dopamine neurotransmission profoundly affects motor, motivation and learning behaviors, and can be observed during the prodromal phase of Parkinson's disease (PD). However, the mechanism underlying these pathophysiological changes remains to be elucidated. Mutations in vacuolar protein sorting 35 (VPS35) and leucine-rich repeat kinase 2 (LRRK2) both lead to autosomal dominant PD, and VPS35 and LRRK2 may physically interact to govern the trafficking of synaptic cargos within the endo-lysosomal network in a kinase-dependent manner. To better understand the functional role of VPS35 and LRRK2 on dopamine physiology, we examined Vps35 haploinsufficient (Haplo) and Vps35 p.D620N knock-in (VKI) mice and how their behavior, dopamine kinetics and biochemistry are influenced by LRRK2 kinase inhibitors. We found Vps35 p.D620N significantly elevates LRRK2-mediated phosphorylation of Rab10, Rab12 and Rab29. In contrast, Vps35 haploinsufficiency reduces phosphorylation of Rab12. While striatal dopamine transporter (DAT) expression and function is similarly impaired in both VKI and Haplo mice, that physiology is normalized in VKI by treatment with the LRRK2 kinase inhibitor, MLi-2. As a corollary, VKI animals show a significant increase in amphetamine induced hyperlocomotion, compared to Haplo mice, that is also abolished by MLi-2. Taken together, these data show Vps35 p.D620N confers a gain-of-function with respect to LRRK2 kinase activity, and that VPS35 and LRRK2 functionally interact to regulate DAT function and striatal dopamine transmission.

In vivo18F-DOPA PET imaging identifies a dopaminergic deficit in a rat model with a G51D α-synuclein mutation

Parkinson's disease (PD) is a neurodegenerative condition with several major hallmarks, including loss of substantia nigra neurons, reduction in striatal dopaminergic function, and formation of α-synuclein-rich Lewy bodies. Mutations in SNCA, encoding for α-synuclein, are a known cause of familial PD, and the G51D mutation causes a particularly aggressive form of the condition. CRISPR/Cas9 technology was used to introduce the G51D mutation into the endogenous rat SNCA gene. SNCA^G51D/+ and SNCA^G51D/G51D rats were born in Mendelian ratios and did not exhibit any severe behavourial defects. L-3,4-dihydroxy-6-¹⁸F-fluorophenylalanine (¹⁸F-DOPA) positron emission tomography (PET) imaging was used to investigate this novel rat model. Wild-type (WT), SNCA^G51D/+ and SNCA^G51D/G51D rats were characterized over the course of ageing (5, 11, and 16 months old) using ¹⁸F-DOPA PET imaging and kinetic modelling. We measured the influx rate constant (K_i) and effective distribution volume ratio (EDVR) of ¹⁸F-DOPA in the striatum relative to the cerebellum in WT, SNCA^G51D/+ and SNCA^G51D/G51D rats. A significant reduction in EDVR was observed in SNCA^G51D/G51D rats at 16 months of age indicative of increased dopamine turnover. Furthermore, we observed a significant asymmetry in EDVR between the left and right striatum in aged SNCA^G51D/G51D rats. The increased and asymmetric dopamine turnover observed in the striatum of aged SNCA^G51D/G51D rats reflects one aspect of prodromal PD, and suggests the presence of compensatory mechanisms. SNCA^G51D rats represent a novel genetic model of PD, and kinetic modelling of ¹⁸F-DOPA PET data has identified a highly relevant early disease phenotype.

A Mouse Model to Test Novel Therapeutics for Parkinson's Disease: an Update on the Thy1-aSyn ("line 61") Mice

Development of neuroprotective therapeutics for Parkinson's disease (PD) is facing a lack of translation from pre-clinical to clinical trials. One strategy for improvement is to increase predictive validity of pre-clinical studies by using extensively characterized animal models with a comprehensive set of validated pharmacodynamic readouts. Mice over-expressing full-length, human, wild-type alpha-synuclein under the Thy-1 promoter (Thy1-aSyn line 61) reproduce key features of sporadic PD, such as progressive loss of striatal dopamine, alpha-synuclein pathology, deficits in motor and non-motor functions, and elevation of inflammatory markers. Extensive work with this model by multiple laboratories over the past decade further increased confidence in its robustness and validity, especially for analyzing pathomechanisms of alpha-synuclein pathology and down-stream pathways, and for pre-clinical drug testing. Interestingly, while postnatal transgene expression is widespread in central and peripheral neurons, the extent and progression of down-stream pathology differs between brain regions, thereby replicating the characteristic selective vulnerability of neurodegenerative diseases. In-depth characterization of these readouts in conjunction with behavioral deficits has led to more informative endpoints for pre-clinical trials. Each drug tested in Thy1-aSyn line 61 enhances knowledge on how molecular targets, pathology, and functional behavioral readouts are interconnected, thereby further optimizing the platform towards predictive validity for clinical trials. Here, we present the current state of the art using Thy1-aSyn line 61 for drug target discovery, validation, and pre-clinical testing.

Aging exacerbates the brain inflammatory micro-environment contributing to α-synuclein pathology and functional deficits in a mouse model of DLB/PD

BACKGROUND

Although ɑ-synuclein (ɑ-syn) spreading in age-related neurodegenerative diseases such as Parkinson's disease (PD) and Dementia with Lewy bodies (DLB) has been extensively investigated, the role of aging in the manifestation of disease remains unclear.

METHODS

We explored the role of aging and inflammation in the pathogenesis of synucleinopathies in a mouse model of DLB/PD initiated by intrastriatal injection of ɑ-syn preformed fibrils (pff).

RESULTS

We found that aged mice showed more extensive accumulation of ɑ-syn in selected brain regions and behavioral deficits that were associated with greater infiltration of T cells and microgliosis. Microglial inflammatory gene expression induced by ɑ-syn-pff injection in young mice had hallmarks of aged microglia, indicating that enhanced age-associated pathologies may result from inflammatory synergy between aging and the effects of ɑ-syn aggregation. Based on the transcriptomics analysis projected from Ingenuity Pathway Analysis, we found a network that included colony stimulating factor 2 (CSF2), LPS related genes, TNFɑ and poly rl:rC-RNA as common regulators.

CONCLUSIONS

We propose that aging related inflammation (eg: CSF2) influences outcomes of pathological spreading of ɑ-syn and suggest that targeting neuro-immune responses might be important in developing treatments for DLB/PD.

Leveraging the preformed fibril model to distinguish between alpha-synuclein inclusion- and nigrostriatal degeneration-associated immunogenicity

Neuroinflammation has become a well-accepted pathologic hallmark of Parkinson's disease (PD). However, it remains unclear whether inflammation, triggered by α-syn aggregation and/or degeneration, contributes to the progression of the disease. Studies examining neuroinflammation in PD are unable to distinguish between Lewy body-associated inflammation and degeneration-associated inflammation, as both pathologies are present simultaneously. Intrastriatal and intranigral injections of alpha-synuclein (α-syn) preformed fibrils (PFFs) results in two distinct pathologic phases: Phase 1: The accumulation and peak formation of α-syn inclusions in nigrostriatal system and, Phase 2: Protracted dopaminergic neuron degeneration. In this review we summarize the current understanding of neuroinflammation in the α-syn PFF model, leveraging the distinct Phase 1 aggregation phase and Phase 2 degeneration phase to guide our interpretations. Studies consistently demonstrate an association between pathologic α-syn aggregation in the substantia nigra (SN) and activation of the innate immune system. Further, major histocompatibility complex-II (MHC-II) antigen presentation is proportionate to inclusion load. The α-syn aggregation phase is also associated with peripheral and adaptive immune cell infiltration to the SN. These findings suggest that α-syn like aggregates are immunogenic and thus have the potential to contribute to the degenerative process. Studies examining neuroinflammation during the neurodegenerative phase reveal elevated innate, adaptive, and peripheral immune cell markers, however limitations of single time point experimental design hinder interpretations as to whether this neuroinflammation preceded, or was triggered by, nigral degeneration. Longitudinal studies across both the aggregation and degeneration phases of the model suggest that microglial activation (MHC-II) is greater in magnitude during the aggregation phase that precedes degeneration. Overall, the consistency between neuroinflammatory markers in the parkinsonian brain and in the α-syn PFF model, combined with the distinct aggregation and degenerative phases, establishes the utility of this model platform to yield insights into pathologic events that contribute to neuroinflammation and disease progression in PD.

Dopaminergic Positron Emission Tomography Imaging in the Alpha-Synuclein Preformed Fibril Model Reveals Similarities to Early Parkinson's Disease

BACKGROUND

Positron emission tomography (PET) imaging in early Parkinson's disease (PD) subjects reveals that increased dopamine (DA) turnover and reduced dopamine transporter (DAT) density precede decreases in DA synthesis and storage. The rat α-synuclein preformed fibril (α-syn PFF) model provides a platform to investigate DA dynamics during multiple stages of α-syn inclusion-triggered nigrostriatal degeneration.

OBJECTIVES

We investigated multiple aspects of in vivo dopaminergic deficits longitudinally and similarities to human PD using translational PET imaging readouts.

METHODS

Longitudinal imaging was performed every 2 months in PFF and control rats for 7 months. [18 F]-Fluoro-3,4-dihydroxyphenyl-L-alanine (FDOPA) imaging was performed to investigate DA synthesis and storage (Kocc ) and DA turnover, estimated by its inverse, the effective distribution volume ratio (EDVR). 11 C-Methylphenidate (MP) was used to estimate DAT density (BPND ).

RESULTS

Early DA turnover increases and DAT binding decreases were observed in the ipsilateral striatum of PFF rats, progressing longitudinally. EDVR decreased 26%, 38%, and 47%, and BPND decreased 36%, 50%, and 65% at the 2-, 4-, and 6-month time points, respectively, compared to ipsilateral control striatum. In contrast, deficits in DA synthesis and storage were not observed in the ipsilateral striatum of PFF rats compared to control injections and were relatively preserved up to 6 months (Kocc decreased 20% at 6 months).

CONCLUSIONS

The relative preservation of DA synthesis and storage compared to robust progressive deficits in DAT density and increases in DA turnover in the rat α-syn PFF model display remarkable face validity to dopaminergic alterations in human PD. © 2022 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

LRRK2 mutant knock-in mouse models: therapeutic relevance in Parkinson's disease

Mutations in the leucine-rich repeat kinase 2 gene (LRRK2) are one of the most frequent genetic causes of both familial and sporadic Parkinson’s disease (PD). Mounting evidence has demonstrated pathological similarities between LRRK2-associated PD (LRRK2-PD) and sporadic PD, suggesting that LRRK2 is a potential disease modulator and a therapeutic target in PD. LRRK2 mutant knock-in (KI) mouse models display subtle alterations in pathological aspects that mirror early-stage PD, including increased susceptibility of nigrostriatal neurotransmission, development of motor and non-motor symptoms, mitochondrial and autophagy-lysosomal defects and synucleinopathies. This review provides a rationale for the use of LRRK2 KI mice to investigate the LRRK2-mediated pathogenesis of PD and implications from current findings from different LRRK2 KI mouse models, and ultimately discusses the therapeutic potentials against LRRK2-associated pathologies in PD.

Cholinergic systems, attentional-motor integration, and cognitive control in Parkinson's disease

Dysfunction and degeneration of CNS cholinergic systems is a significant component of multi-system pathology in Parkinson's disease (PD). We review the basic architecture of human CNS cholinergic systems and the tools available for studying changes in human cholinergic systems. Earlier post-mortem studies implicated abnormalities of basal forebrain corticopetal cholinergic (BFCC) and pedunculopontine-laterodorsal tegmental (PPN-LDT) cholinergic projections in cognitive deficits and gait-balance deficits, respectively. Recent application of imaging methods, particularly molecular imaging, allowed more sophisticated correlation of clinical features with regional cholinergic deficits. BFCC projection deficits correlate with general and domain specific cognitive deficits, particularly for attentional and executive functions. Detailed analyses suggest that cholinergic deficits within the salience and cingulo-opercular task control networks, including both neocortical, thalamic, and striatal nodes, are a significant component of cognitive deficits in non-demented PD subjects. Both BFCC and PPN-LDT cholinergic projection systems, and striatal cholinergic interneuron (SChI), abnormalities are implicated in PD gait-balance disorders. In the context of experimental studies, these results indicate that disrupted attentional functions of BFCC and PPN-LDT cholinergic systems underlie impaired gait-balance functions. SChI dysfunction likely impairs intra-striatal integration of attentional and motor information. Thalamic and entorhinal cortex cholinergic deficits may impair multi-sensory integration. Overt degeneration of CNS systems may be preceded by increased activity of cholinergic neurons compensating for nigrostriatal dopaminergic deficits. Subsequent dysfunction and degeneration of cholinergic systems unmasks and exacerbates functional deficits secondary to dopaminergic denervation. Research on CNS cholinergic systems dysfunctions in PD requires a systems-level approach to understanding PD pathophysiology.

Deletion of lrrk2 causes early developmental abnormalities and age-dependent increase of monoamine catabolism in the zebrafish brain

LRRK2 gain-of-function is considered a major cause of Parkinson's disease (PD) in humans. However, pathogenicity of LRRK2 loss-of-function in animal models is controversial. Here we show that deletion of the entire zebrafish lrrk2 locus elicits a pleomorphic transient brain phenotype in maternal-zygotic mutant embryos (mzLrrk2). In contrast to lrrk2, the paralog gene lrrk1 is virtually not expressed in the brain of both wild-type and mzLrrk2 fish at different developmental stages. Notably, we found reduced catecholaminergic neurons, the main target of PD, in specific cell populations in the brains of mzLrrk2 larvae, but not adult fish. Strikingly, age-dependent accumulation of monoamine oxidase (MAO)-dependent catabolic signatures within mzLrrk2 brains revealed a previously undescribed interaction between LRRK2 and MAO biological activities. Our results highlight mzLrrk2 zebrafish as a tractable tool to study LRRK2 loss-of-function in vivo, and suggest a link between LRRK2 and MAO, potentially of relevance in the prodromic stages of PD.

The C-Terminal Domain of LRRK2 with the G2019S Substitution Increases Mutant A53T α-Synuclein Toxicity in Dopaminergic Neurons In Vivo

Alpha-synuclein (α-syn) and leucine-rich repeat kinase 2 (LRRK2) play crucial roles in Parkinson's disease (PD). They may functionally interact to induce the degeneration of dopaminergic (DA) neurons via mechanisms that are not yet fully understood. We previously showed that the C-terminal portion of LRRK2 (ΔLRRK2) with the G2019S mutation (ΔLRRK2^G2019S) was sufficient to induce neurodegeneration of DA neurons in vivo, suggesting that mutated LRRK2 induces neurotoxicity through mechanisms that are (i) independent of the N-terminal domains and (ii) "cell-autonomous". Here, we explored whether ΔLRRK2^G2019S could modify α-syn toxicity through these two mechanisms. We used a co-transduction approach in rats with AAV vectors encoding ΔLRRK2^G2019S or its "dead" kinase form, ΔLRRK2^DK, and human α-syn with the A53T mutation (AAV-α-syn^A53T). Behavioral and histological evaluations were performed at 6- and 15-weeks post-injection. Results showed that neither form of ΔLRRK2 alone induced the degeneration of neurons at these post-injection time points. By contrast, injection of AAV-α-syn^A53T alone resulted in motor signs and degeneration of DA neurons. Co-injection of AAV-α-syn^A53T with AAV-ΔLRRK2^G2019S induced DA neuron degeneration that was significantly higher than that induced by AAV-α-syn^A53T alone or with AAV-ΔLRRK2^DK. Thus, mutated α-syn neurotoxicity can be enhanced by the C-terminal domain of LRRK2^G2019 alone^, through cell-autonomous mechanisms.

Writers Are Common among Parkinson's Disease Patients: A Longitudinal Study

Parkinson's disease (PD) patients may have a specific personality profile, which includes being introvert, cautious and devoted to hard work. The evaluation of psychological characteristics must be evaluated according to methods for assessments of personality disorders. Such evaluations are often time-consuming and available only in research settings. The “parkinsonian trait” may be established early in life but may change with disease progression. To overcome this long interval before onset of PD questions on literary activities were included in the medical record. Three percent of PD patients could be defined as writers, significantly higher than observed in the general population. PD writers published their first books long before onset of disease. Being a writer is an extrovert trait meaning that the patient is prepared for criticism and publicity. We suggest that questions regarding personal activities prior to disease onset add valuable information on personality which differs significantly from traits observed later in the disease period.

LRRK2 G2019S kinase activity triggers neurotoxic NSF aggregation

Parkinson's disease is characterized by the progressive degeneration of dopaminergic neurons within the substantia nigra pars compacta and the presence of protein aggregates in surviving neurons. The LRRK2 G2019S mutation is one of the major determinants of familial Parkinson's disease cases and leads to late-onset Parkinson's disease with pleomorphic pathology, including α-synuclein accumulation and deposition of protein inclusions. We demonstrated that LRRK2 phosphorylates N-ethylmaleimide sensitive factor (NSF). We observed aggregates containing NSF in basal ganglia specimens from patients with Parkinson's disease carrying the G2019S variant, and in cellular and animal models expressing the LRRK2 G2019S variant. We found that LRRK2 G2019S kinase activity induces the accumulation of NSF in toxic aggregates. Of note, the induction of autophagy cleared NSF aggregation and rescued motor and cognitive impairment observed in aged hG2019S bacterial artificial chromosome (BAC) mice. We suggest that LRRK2 G2019S pathological phosphorylation impacts on NSF biochemical properties, thus causing the formation of cytotoxic protein inclusions.

Dynamic control of the dopamine transporter in neurotransmission and homeostasis

The dopamine transporter (DAT) transports extracellular dopamine into the intracellular space contributing to the regulation of dopamine neurotransmission. A reduction of DAT density is implicated in Parkinson's disease (PD) by neuroimaging; dopamine turnover is dopamine turnover is elevated in early symptomatic PD and in presymptomatic individuals with monogenic mutations causal for parkinsonism. As an integral plasma membrane protein, DAT surface expression is dynamically regulated through endocytic trafficking, enabling flexible control of dopamine signaling in time and space, which in turn critically modulates movement, motivation and learning behavior. Yet the cellular machinery and functional implications of DAT trafficking remain enigmatic. In this review we summarize mechanisms governing DAT trafficking under normal physiological conditions and discuss how PD-linked mutations may disturb DAT homeostasis. We highlight the complexity of DAT trafficking and reveal DAT dysregulation as a common theme in genetic models of parkinsonism.

Brain Microglial Activation Increased in Glucocerebrosidase (GBA) Mutation Carriers without Parkinson's disease

BACKGROUND

Glucocerebrosidase gene mutations are a common genetic risk factor for Parkinson's disease. They exhibit incomplete penetrance. The objective of the present study was to measure microglial activation and dopamine integrity in glucocerebrosidase gene mutation carriers without Parkinson's disease compared to controls.

METHODS

We performed PET scans on 9 glucocerebrosidase gene mutation carriers without Parkinson's disease and 29 age-matched controls. We measured microglial activation as 11 C-(R)-PK11195 binding potentials, and dopamine terminal integrity with 18 F-dopa influx constants.

RESULTS

The 11 C-(R)-PK11195 binding potential was increased in the substantia nigra of glucocerebrosidase gene carriers compared with controls (Student t test; right, t = -4.45, P = 0.0001). Statistical parametric mapping also localized significantly increased 11 C-(R)-PK11195 binding potential in the occipital and temporal lobes, cerebellum, hippocampus, and mesencephalon. The degree of hyposmia correlated with nigral 11 C-(R)-PK11195 regional binding potentials (Spearman's rank, P = 0.0066). Mean striatal 18 F-dopa uptake was similar to healthy controls.

CONCLUSIONS

In vivo 11 C-(R)-PK11195 PET imaging detects neuroinflammation in brain regions susceptible to Lewy pathology in glucocerebrosidase gene mutation carriers without Parkinson's. © 2020 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

LRRK2 at the pre-synaptic site: A 16-years perspective

Parkinson's disease is a common neurodegenerative disorder and is clinically characterized by bradykinesia, rigidity, and resting tremor. Missense mutations in the leucine-rich repeat protein kinase-2 gene (LRRK2) are a recognized cause of inherited Parkinson's disease. The physiological and pathological impact of LRRK2 is still obscure, but accumulating evidence indicates that LRRK2 orchestrates diverse aspects of membrane trafficking, such as membrane fusion and vesicle formation and transport along actin and tubulin tracks. In the present review, we focus on the special relation between LRRK2 and synaptic vesicles. LRRK2 binds and phosphorylates key actors within the synaptic vesicle cycle. Accordingly, alterations in dopamine and glutamate transmission have been described upon LRRK2 manipulations. However, the different modeling strategies and phenotypes observed require a critical approach to decipher the outcome of LRRK2 at the pre-synaptic site.

Inflammatory Diseases Among Norwegian LRRK2 Mutation Carriers. A 15-Years Follow-Up of a Cohort

The first families with LRRK2 related Parkinson’s disease (PD) were presented around 15 years ago and numerous papers have described the characteristics of the LRRK2 phenotype. The prevalence of autosomal dominant PD varies around the world mainly depending on local founder effects. The highest prevalence of LRRK2 G2019S PD in Norway is located to the central part of the country and most families could be traced back to common ancestors. The typical Norwegian LRRK2 phenotype is not different from classical PD and similar to that seen in most other LRRK2 families. The discovery of LRRK2 PD has allowed us to follow-up multi-incident families and to study their phenotype longitudinally. In the Norwegian LRRK2 families there has been a significantly higher incidence of inflammatory diseases like multiple sclerosis and rheumatoid arthritis that seen in other PD populations. Recent studies in LRRK2 mechanisms have indicated that this protein may be crucial in initiating disease processes. In this short survey of 100 Norwegian mutation carriers followed through more than 15 years are presented. The prevalence of inflammatory diseases among these cases is highlighted. The role of LRRK2 in the conversion process from carrier status to PD phenotype is still unknown and disease generating mechanisms important for initiating LRRK2 PD are still to be identified.

11C- and 18F-Radiotracers for In Vivo Imaging of the Dopamine System: Past, Present and Future

The applications of positron emission tomography (PET) imaging to study brain biochemistry, and in particular the aspects of dopamine neurotransmission, have grown significantly over the 40 years since the first successful in vivo imaging studies in humans. In vivo PET imaging of dopaminergic functions of the central nervous system (CNS) including dopamine synthesis, vesicular storage, synaptic release and receptor binding, and reuptake processes, are now routinely used for studies in neurology, psychiatry, drug abuse and addiction, and drug development. Underlying these advances in PET imaging has been the development of the unique radiotracers labeled with positron-emitting radionuclides such as carbon-11 and fluorine-18. This review focuses on a selection of the more accepted and utilized PET radiotracers currently available, with a look at their past, present and future.

Serotonergic System Impacts Levodopa Response in Early Parkinson's and Future Risk of Dyskinesia

BACKGROUND

The serotonergic system is known to contribute to levodopa-derived dopamine release in advanced Parkinson's disease.

OBJECTIVE

We investigated the role of the serotonergic system in determining response to treatment in early disease and risk for complications concurrently with dopaminergic alterations.

METHODS

Eighteen patients with early and stable Parkinson's disease underwent multitracer positron emission tomography using [¹¹ C]dihydrotetrabenazine (vesicular monoamine transporter 2 marker), [¹¹ C]methylphenidate (dopamine transporter marker), [¹¹ C]-3-amino-4-(2-dimethylaminomethylphenylsulfanyl)-benzonitrile (DASB, serotonin transporter marker), and [¹¹ C]raclopride (D2 marker) to investigate relationships between striatal dopaminergic and serotonergic alterations and levodopa-induced dopamine release, related to motor response to treatment and risk for dyskinesias, using a novel joint pattern analysis.

RESULTS

The joint pattern analysis revealed correlated spatial patterns conceptually related to abnormal dopamine turnover in the putamen (higher dopamine release associated with dopaminergic and serotonergic denervation); response to treatment significantly inversely correlated with turnover-related dopamine release (P < 10^-5 ). Patterns identified without inclusion of the DASB data showed no correlation with clinical data, indicating an important contribution from the serotonergic system to a clinically relevant abnormal dopamine release in early disease. Subjects who experienced dyskinesia 3 years after baseline scans showed higher turnover-related dopamine release compared with subjects who remained stable (P < 0.01).

CONCLUSIONS

Joint analysis of dopaminergic and serotonergic data identified a turnover-related dopamine release component, strongly related to motor response to levodopa in early disease and contributing to higher risk for dyskinesia. These findings suggest that the contribution of the serotonergic system to dopamine release not only increases the risk for motor complications but also fails to provide sustained therapeutic advantage in early disease. © 2020 International Parkinson and Movement Disorder Society.

Neuroimaging Advances in Parkinson's Disease and Atypical Parkinsonian Syndromes

Parkinson's disease (PD) and atypical Parkinsonian syndromes are progressive heterogeneous neurodegenerative diseases that share clinical characteristic of parkinsonism as a common feature, but are considered distinct clinicopathological disorders. Based on the predominant protein aggregates observed within the brain, these disorders are categorized as, (1) α-synucleinopathies, which include PD and other Lewy body spectrum disorders as well as multiple system atrophy, and (2) tauopathies, which comprise progressive supranuclear palsy and corticobasal degeneration. Although, great strides have been made in neurodegenerative disease research since the first medical description of PD in 1817 by James Parkinson, these disorders remain a major diagnostic and treatment challenge. A valid diagnosis at early disease stages is of paramount importance, as it can help accommodate differential prognostic and disease management approaches, enable the elucidation of reliable clinicopathological relationships ideally at prodromal stages, as well as facilitate the evaluation of novel therapeutics in clinical trials. However, the pursuit for early diagnosis in PD and atypical Parkinsonian syndromes is hindered by substantial clinical and pathological heterogeneity, which can influence disease presentation and progression. Therefore, reliable neuroimaging biomarkers are required in order to enhance diagnostic certainty and ensure more informed diagnostic decisions. In this article, an updated presentation of well-established and emerging neuroimaging biomarkers are reviewed from the following modalities: (1) structural magnetic resonance imaging (MRI), (2) diffusion-weighted and diffusion tensor MRI, (3) resting-state and task-based functional MRI, (4) proton magnetic resonance spectroscopy, (5) transcranial B-mode sonography for measuring substantia nigra and lentiform nucleus echogenicity, (6) single photon emission computed tomography for assessing the dopaminergic system and cerebral perfusion, and (7) positron emission tomography for quantifying nigrostriatal functions, glucose metabolism, amyloid, tau and α-synuclein molecular imaging, as well as neuroinflammation. Multiple biomarkers obtained from different neuroimaging modalities can provide distinct yet corroborative information on the underlying neurodegenerative processes. This integrative "multimodal approach" may prove superior to single modality-based methods. Indeed, owing to the international, multi-centered, collaborative research initiatives as well as refinements in neuroimaging technology that are currently underway, the upcoming decades will mark a pivotal and exciting era of further advancements in this field of neuroscience.

Strategic Timing of Glial HMOX1 Expression Results in Either Schizophrenia-Like or Parkinsonian Behavior in Mice

Aims: In this original research communication, we assess the impact of shifting the window of glial HMOX1 overexpression in mice from early-to-midlife to mid-to-late life, resulting in two disparate conditions modeling schizophrenia (SCZ) and Parkinson's disease (PD). Mesolimbic hyperdopaminergia is a widely accepted feature of SCZ, while nigrostriatal hypodopaminergia is the sine qua non of idiopathic PD. Although the advent of parkinsonian features in SCZ patients after treatment with antidopaminergic agents is intuitive, subtle features of parkinsonism commonly observed in young, drug-naïve schizophrenics are not. Similarly, emergent psychosis in PD subjects receiving levodopa replacement is not unusual, whereas spontaneous hallucinosis in nonmedicated persons with idiopathic PD is enigmatic. Investigations using GFAP.HMOX1 mice may shed light on these clinical paradoxes. Results: Astroglial heme oxygenase-1 (HO-1) overexpression in mice throughout embryogenesis until 6 or 12 months of age resulted in hyperdopaminergia, hyperkinesia/stereotypy ameliorated with clozapine, deficient prepulse inhibition of the acoustic startle response, reduced preference for social novelty, impaired nest building, and cognitive dysfunction reminiscent of SCZ. On the contrary, astroglial HO-1 overexpression between 8.5 and 19 months of age yielded a PD-like behavioral phenotype with hypodopaminergia, altered gait, locomotor incoordination, and reduced olfaction. Innovation: We conjecture that region-specific disparities in the susceptibility of dopaminergic and other circuitry to the trophic and degenerative influences of glial HMOX1 induction may permit the concomitant expression of mixed SCZ and PD traits within affected individuals. Conclusion: Elucidation of these converging mechanisms may (i) help better understand disease pathogenesis and (ii) identify HO-1 as a potential therapeutic target in neurodevelopmental and neurodegenerative disorders.

What Have We Learned from Cerebrospinal Fluid Studies about Biomarkers for Detecting LRRK2 Parkinson's Disease Patients and Healthy Subjects with Parkinson's-Associated LRRK2 Mutations?

Mutations in the leucine-rich repeat kinase 2 (LRRK2) gene are the most common known cause of autosomal dominant Parkinson’s disease (PD) and sporadic PD (sPD). The clinical presentation of LRRK2 PD is similar to sPD, and except for genetic testing, no biochemical or imaging markers can differentiate LRRK2 PD from sPD. Discovery of such biomarkers could indicate neuropathological mechanisms that are unique to or increased in LRRK2 PD. This review discusses findings in 17 LRRK2 - related CSF studies found on PubMed. Most of these studies compared analyte concentrations between four diagnostic groups: LRRK2 PD patients, sPD patients, asymptomatic control subjects carrying PD-associated LRRK2 mutations (LRRK2 CTL), and healthy control subjects lacking LRRK2 mutations (CTL). Analytes examined in these studies included Aβ1-42, tau, α-synuclein, oxidative stress markers, autophagy-related proteins, pteridines, neurotransmitter metabolites, exosomal LRRK2 protein, RNA species, inflammatory cytokines, mitochondrial DNA (mtDNA), and intermediary metabolites. FINDINGS: Pteridines, α-synuclein, mtDNA, 5-hydroxyindolacetic acid, β-D-glucose, lamp2, interleukin-8, and vascular endothelial growth factor were suggested to differentiate LRRK2 PD from sPD patients; 8-hydroxy-2’-deoxyguanosine (8-OHdG), 8-isoprostane (8-ISO), 2-hydroxybutyrate, mtDNA, lamp2, and neopterin may differentiate between LRRK2 CTL and LRRK2 PD subjects; and soluble oligomeric α-synuclein, 8-OHdG, and 8-ISO might differentiate LRRK2 CTL from CTL subjects. CONCLUSIONS: The low numbers of investigations of each analyte, small sample sizes, and methodological differences limit conclusions that can be drawn from these studies. Further investigations are indicated to determine the validity of the analytes identified in these studies as possible biomarkers for LRRK2 PD patients and/or LRRK2 CTL subjects.

Long-Term Outcomes of Genetic Parkinson's Disease

Parkinson’s disease (PD) is a progressive neurodegenerative disorder that affects 1–2% of people by the age of 70 years. Age is the most important risk factor, and most cases are sporadic without any known environmental or genetic causes. Since the late 1990s, mutations in the genes SNCA, PRKN, LRRK2, PINK1, DJ-1, VPS35, and GBA have been shown to be important risk factors for PD. In addition, common variants with small effect sizes are now recognized to modulate the risk for PD. Most studies in genetic PD have focused on finding new genes, but few have studied the long-term outcome of patients with the specific genetic PD forms. Patients with known genetic PD have now been followed for more than 20 years, and we see that they may have distinct and different prognoses. New therapeutic possibilities are emerging based on the genetic cause underlying the disease. Future medication may be based on the pathophysiology individualized to the patient’s genetic background. The challenge is to find the biological consequences of different genetic variants. In this review, the clinical patterns and long-term prognoses of the most common genetic PD variants are presented.

Single Inflammatory Trigger Leads to Neuroinflammation in LRRK2 Rodent Model without Degeneration of Dopaminergic Neurons

BACKGROUND

Leucine-rich repeat kinase 2 (LRRK2) mutations are the most common genetic risk factor for Parkinson's disease (PD). While the corresponding pathogenic mechanisms remain largely unknown, LRRK2 has been implicated in the immune system.

OBJECTIVE

To assess whether LRRK2 mutations alter the sensitivity to a single peripheral inflammatory trigger, with ultimate impact on dopaminergic integrity, using a longitudinal imaging-based study design.

METHODS

Rats carrying LRRK2 p.G2019S and non-transgenic (NT) littermates were treated peripherally with lipopolysaccharide (LPS). They were monitored over 10 months with PET markers for neuroinflammation and dopaminergic integrity, and with behavioral testing. Tyrosine hydroxylase and CD68 expression were assessed postmortem, 12 months after LPS treatment, in the striatum and substantia nigra.

RESULTS

Longitudinal [11C]PBR28 PET imaging revealed that LPS treatment caused inflammation in the brain, increasing over time, as compared to saline (corrected p = 0.008). LPS treated LRRK2 animals exhibited significantly increased neuroinflammation in the cortex and ventral-regions compared to saline treated animals (LRRK2 and NT) at 10 months post treatment, with the increase in [11C]PBR28 binding from baseline averaging 0.128±0.045 g/mL. For LPS treated NT animals, the increase was not significant. CD68 immunohistochemistry data supported the imaging results, but without reaching statistical significance. No dopaminergic degeneration was observed.

CONCLUSION

A single peripheral inflammatory trigger elicited long lasting, progressive neuroinflammation. A trend for an exacerbated inflammatory response in LRRK2 animals compared to NT controls was observed. Translationally, this implies that repeated exposure to inflammatory triggers may be needed for LRRK2 mutation carriers to develop active PD.

Clinical and Imaging Markers of Prodromal Parkinson's Disease

The diagnosis of Parkinson's disease (PD) relies on the clinical effects of dopamine deficiency, including bradykinesia, rigidity and tremor, usually manifesting asymmetrically. Misdiagnosis is common, due to overlap of symptoms with other neurodegenerative disorders such as multiple system atrophy and progressive supranuclear palsy, and only autopsy can definitively confirm the disease. Motor deficits generally appear when 50–60% of dopaminergic neurons in the substantia nigra are already lost, limiting the effectiveness of potential neuroprotective therapies. Today, we consider PD to be not just a movement disorder, but rather a complex syndrome non-motor symptoms (NMS) including disorders of sleep-wake cycle regulation, cognitive impairment, disorders of mood and affect, autonomic dysfunction, sensory symptoms and pain. Symptomatic LRRK2 mutation carriers share non-motor features with individuals with sporadic PD, including hyposmia, constipation, impaired color discrimination, depression, and sleep disturbance. Following the assumption that the pre-symptomatic gene mutation carriers will eventually exhibit clinical symptoms, their neuroimaging results can be extended to the pre-symptomatic stage of PD. The long latent phase of PD, termed prodromal-PD, represents an opportunity for early recognition of incipient PD. Early recognition could allow initiation of possible neuroprotective therapies at a stage when therapies might be most effective. The number of markers with the sufficient level of evidence to be included in the MDS research criteria for prodromal PD have increased during the last 10 years. Here, we review the approach to prodromal PD, with an emphasis on clinical and imaging markers and report results from our neuroimaging study, a retrospective evaluation of a cohort of 39 participants who underwent DAT-SPECT scan as part of their follow up. The study was carried out to see if it was possible to detect subclinical signs in the preclinical (neurodegenerative processes have commenced, but there are no evident symptoms or signs) and prodromal (symptoms and signs are present, but are yet insufficient to define disease) stages of PD.

Olfactory dysfunction correlates with putaminal dopamine turnover in early de novo Parkinson's disease

Although olfactory dysfunction is one of the most well-established prodromal symptoms in Parkinson's disease (PD), its correlation with clinical disease progression or dopaminergic dysfunction still remains unclear. We here evaluated the association of striatal dopamine metabolism and olfactory function in a homogenous cohort of 30 patients with early untreated de novo PD. Striatal dopamine metabolism was assessed by the extended ¹⁸Fluorodopa PET scanning protocol to measure ¹⁸Fluorodopa uptake (K_occ) and the effective dopamine distribution volume ratio (EDVR) as the inverse of dopamine turnover. Olfactory function was estimated by the "Sniffin' Sticks" test including odor threshold (T), discrimination (D) and identification (I) assessment. We detected moderate correlations of the EDVR in the posterior putamen with the TDI composite score (r = 0.412; p = 0.024; Pearson's correlation test) and the odor identification score (r = 0.444; p = 0.014). These correlations were confirmed by multivariate regression analyses using age, sex, symptom duration and disease severity as measured by UPDRSIII motor score as candidate covariates. No other associations were observed between olfaction measures and K_occ and EDVR in all striatal regions. Together, olfactory dysfunction in early PD is not correlated with striatal ¹⁸Fluorodopa uptake as a measure for dopaminergic degeneration, but with putaminal dopamine turnover as a marker for dopaminergic presynaptic compensatory processes in early PD. These results should be treated as hypothesis generating and require confirmation by larger multicenter studies.

Brain tyrosinase overexpression implicates age-dependent neuromelanin production in Parkinson's disease pathogenesis

In Parkinson's disease (PD) there is a selective degeneration of neuromelanin-containing neurons, especially substantia nigra dopaminergic neurons. In humans, neuromelanin accumulates with age, the latter being the main risk factor for PD. The contribution of neuromelanin to PD pathogenesis remains unknown because, unlike humans, common laboratory animals lack neuromelanin. Synthesis of peripheral melanins is mediated by tyrosinase, an enzyme also present at low levels in the brain. Here we report that overexpression of human tyrosinase in rat substantia nigra results in age-dependent production of human-like neuromelanin within nigral dopaminergic neurons, up to levels reached in elderly humans. In these animals, intracellular neuromelanin accumulation above a specific threshold is associated to an age-dependent PD phenotype, including hypokinesia, Lewy body-like formation and nigrostriatal neurodegeneration. Enhancing lysosomal proteostasis reduces intracellular neuromelanin and prevents neurodegeneration in tyrosinase-overexpressing animals. Our results suggest that intracellular neuromelanin levels may set the threshold for the initiation of PD.

Brain tyrosinase overexpression implicates age-dependent neuromelanin production in Parkinson's disease pathogenesis

In Parkinson's disease (PD) there is a selective degeneration of neuromelanin-containing neurons, especially substantia nigra dopaminergic neurons. In humans, neuromelanin accumulates with age, the latter being the main risk factor for PD. The contribution of neuromelanin to PD pathogenesis remains unknown because, unlike humans, common laboratory animals lack neuromelanin. Synthesis of peripheral melanins is mediated by tyrosinase, an enzyme also present at low levels in the brain. Here we report that overexpression of human tyrosinase in rat substantia nigra results in age-dependent production of human-like neuromelanin within nigral dopaminergic neurons, up to levels reached in elderly humans. In these animals, intracellular neuromelanin accumulation above a specific threshold is associated to an age-dependent PD phenotype, including hypokinesia, Lewy body-like formation and nigrostriatal neurodegeneration. Enhancing lysosomal proteostasis reduces intracellular neuromelanin and prevents neurodegeneration in tyrosinase-overexpressing animals. Our results suggest that intracellular neuromelanin levels may set the threshold for the initiation of PD.

Chronic nicotine improves cognitive and social impairment in mice overexpressing wild type α-synuclein

In addition to dopaminergic and motor deficits, patients with Parkinson's disease (PD) suffer from non-motor symptoms, including early cognitive and social impairment, that do not respond well to dopaminergic therapy. Cholinergic deficits may contribute to these problems, but cholinesterase inhibitors have limited efficacy. Mice over-expressing α-synuclein, a protein critically associated with PD, show deficits in cognitive and social interaction tests, as well as a decrease in cortical acetylcholine. We have evaluated the effects of chronic administration of nicotine in mice over-expressing wild type human α-synuclein under the Thy1-promoter (Thy1-aSyn mice). Nicotine was administered subcutaneously by osmotic minipump for 6 months from 2 to 8 months of age at 0.4 mg/kg/h and 2.0 mg/kg/h. The higher dose was toxic in the Thy1-aSyn mice, but the low dose was well tolerated and both doses ameliorated cognitive impairment in Y-maze performance after 5 months of treatment. In a separate cohort of Thy1-aSyn mice, nicotine was administered at the lower dose for one month beginning at 5 months of age. This treatment partially eliminated the cognitive deficit in novel object recognition and social impairment. In contrast, chronic nicotine did not improve motor deficits after 2, 4 or 6 months of treatment, nor modified α-synuclein aggregation, tyrosine hydroxylase immunostaining, synaptic and dendritic markers, or microglial activation in Thy1-aSyn mice. These results suggest that cognitive and social impairment in synucleinopathies like PD may result from deficits in cholinergic neurotransmission and may benefit from chronic administration of nicotinic agonists.

Altered dopamine release and monoamine transporters in Vps35 p.D620N knock-in mice

Vacuolar protein sorting 35 (VPS35) is a core component of the retromer trimer required for endosomal membrane-associated protein trafficking. The discovery of a missense mutation, Vps35 p.D620N implicates retromer dysfunction in the pathogenesis of Parkinson's disease (PD). We have characterized a knock-in mouse with a Vps35 p.D620N substitution (hereafter referred to as VKI) at 3 months of age. Standardized behavioral testing did not observe overt movement disorder. Tyrosine hydroxylase (TH)-positive nigral neuron counts and terminal expression in striata were comparable across genotypes. Fast scan cyclic voltammetry revealed increased dopamine release in VKI striatal slices. While extracellular dopamine collected via striatal microdialysis of freely moving animals was comparable across genotypes, the ratio of dopamine metabolites to dopamine suggests increased dopamine turnover in VKI homozygous mice. Western blot of striatal proteins revealed a genotype-dependent decrease in dopamine transporter (DAT) along with an increase in vesicular monoamine transporter 2 (VMAT2), albeit independent of changes in other synaptic markers. The reduction in DAT was further supported by immunohistochemical analysis. The data show that the dopaminergic system of VKI mice is profoundly altered relative to wild-type littermates. We conclude early synaptic dysfunction contributes to age-related pathophysiology in the nigrostriatal system that may lead to parkinsonism in man.

Investigation of serotonergic Parkinson's disease-related covariance pattern using [11C]-DASB/PET

We used positron emission tomography imaging with [11C]-3-amino-4-(2-dimethylaminomethylphenylsulfanyl)- benzonitrile (DASB) and principal component analysis to investigate whether a specific Parkinson's disease (PD)-related spatial covariance pattern could be identified for the serotonergic system. We also explored if non-manifesting leucine-rich repeat kinase 2 (LRRK2) mutation carriers, with normal striatal dopaminergic innervation as measured with [11C]-dihydrotetrabenazine (DTBZ), exhibit a distinct spatial covariance pattern compared to healthy controls and subjects with manifest PD. 15 subjects with sporadic PD, eight subjects with LRRK2 mutation-associated PD, nine LRRK2 non-manifesting mutation carriers, and nine healthy controls participated in the study. The analysis was applied to the DASB non-displaceable binding potential values evaluated in 42 pre-defined regions of interest. PD was found to be associated with a specific spatial covariance pattern, comprising relatively decreased DASB binding in the caudate, putamen and substantia nigra and relatively preserved binding in the hypothalamus and hippocampus; the expression of this pattern in PD subjects was significantly higher than in healthy controls (P < 0.001) and correlated significantly with disease duration (P < 0.01) and with DTBZ binding in the more affected putamen (P < 0.01). The LRRK2 non-manifesting mutation carriers expressed a different pattern, also significantly different from healthy controls (P < 0.001), comprising relatively decreased DASB binding in the pons, pedunculopontine nucleus, thalamus and rostral raphe nucleus, and with relatively preserved binding in the hypothalamus, amygdala, hippocampus and substantia nigra. This pattern was not present in either sporadic or LRRK2 mutation-associated PD subjects. These findings, although obtained with a relatively limited number of subjects, suggest that specific and overall distinct spatial serotonergic patterns may be associated with PD and LRRK2 mutations. Alterations in regions where relative upregulation is observed in both patterns may be indicative of compensatory mechanisms preceding or protecting from disease manifestation.

The effect of LRRK2 mutations on the cholinergic system in manifest and premanifest stages of Parkinson's disease: a cross-sectional PET study

BACKGROUND

Markers of neuroinflammation are increased in some patients with LRRK2 Parkinson's disease compared with individuals with idiopathic Parkinson's disease, suggesting possible differences in disease pathogenesis. Previous PET studies have suggested amplified dopamine turnover and preserved serotonergic innervation in LRRK2 mutation carriers. We postulated that patients with LRRK2 mutations might show abnormalities of central cholinergic activity, even before the diagnosis of Parkinson's disease.

METHODS

Between June, 2009, and December, 2015, we recruited participants from four movement disorder clinics in Canada, Norway, and the USA. Patients with Parkinson's disease were diagnosed by movement disorder neurologists on the basis of the UK Parkinson's Disease Society Brain Bank criteria. LRRK2 carrier status was confirmed by bidirectional Sanger sequencing. We used the PET tracer N-¹¹C-methyl-piperidin-4-yl propionate to scan for acetylcholinesterase activity. The primary outcome measure was rate of acetylcholinesterase hydrolysis, calculated using the striatal input method. We compared acetylcholinesterase hydrolysis rates between groups using ANCOVA, with adjustment for age based on the results of linear regression analysis.

FINDINGS

We recruited 14 patients with LRRK2 Parkinson's disease, 16 LRRK2 mutation carriers without Parkinson's disease, eight patients with idiopathic Parkinson's disease, and 11 healthy controls. We noted significant between-group differences in rates of acetylcholinesterase hydrolysis in cortical regions (average cortex p=0·009, default mode network-related regions p=0·006, limbic network-related regions p=0·020) and the thalamus (p=0·008). LRRK2 mutation carriers without Parkinson's disease had increased acetylcholinesterase hydrolysis rates compared with healthy controls in the cortex (average cortex, p=0·046). Patients with LRRK2 Parkinson's disease had significantly higher acetylcholinesterase activity in some cortical regions (average cortex p=0·043, default mode network-related regions p=0·021) and the thalamus (thalamus p=0·004) compared with individuals with idiopathic disease. Acetylcholinesterase hydrolysis rates in healthy controls were correlated inversely with age.

INTERPRETATION

LRRK2 mutations are associated with significantly increased cholinergic activity in the brain in mutation carriers without Parkinson's disease compared with healthy controls and in LRRK2 mutation carriers with Parkinson's disease compared with individuals with idiopathic disease. Changes in cholinergic activity might represent early and sustained attempts to compensate for LRRK2-related dysfunction, or alteration of acetylcholinesterase in non-neuronal cells.

FUNDING

Michael J Fox Foundation, National Institutes of Health, and Pacific Alzheimer Research Foundation.

Update on Molecular Imaging in Parkinson's Disease

Advances in radionuclide tracers have allowed for more accurate imaging that reflects the actions of numerous neurotransmitters, energy metabolism utilization, inflammation, and pathological protein accumulation. All of these achievements in molecular brain imaging have broadened our understanding of brain function in Parkinson's disease (PD). The implementation of molecular imaging has supported more accurate PD diagnosis as well as assessment of therapeutic outcome and disease progression. Moreover, molecular imaging is well suited for the detection of preclinical or prodromal PD cases. Despite these advances, future frontiers of research in this area will focus on using multi-modalities combining positron emission tomography and magnetic resonance imaging along with causal modeling with complex algorithms.

Is Axonal Degeneration a Key Early Event in Parkinson's Disease?

Recent research suggests that in Parkinson's disease the long, thin and unmyelinated axons of dopaminergic neurons degenerate early in the disease process. We organized a workshop entitled 'Axonal Pathology in Parkinson's disease', on March 23rd, 2016, in Cleveland, Ohio with the goals of summarizing the state-of-the-art and defining key gaps in knowledge. A group of eight research leaders discussed new developments in clinical pathology, functional imaging, animal models, and mechanisms of degeneration including neuroinflammation, autophagy and axonal transport deficits. While the workshop focused on PD, comparisons were made to other neurological conditions where axonal degeneration is well recognized.

Neuroimaging and clinical features in adults with a 22q11.2 deletion at risk of Parkinson's disease

The recurrent 22q11.2 deletion is a genetic risk factor for early-onset Parkinson's disease. Adults with the associated 22q11.2 deletion syndrome (22q11.2DS) may exhibit phenotypes that could help identify those at highest risk and reveal disease trajectories. We investigated clinical and neuroimaging features relevant to Parkinson's disease in 26 adults: 13 with 22q11.2DS at genetic risk of Parkinson's disease (mean age = 41.5 years, standard deviation = 9.7), 12 healthy age and sex-matched controls, and a 22q11.2DS patient with l-DOPA-responsive early-onset Parkinson's disease. Neuroimaging included transcranial sonography and positron emission tomography using 11C-dihydrotetrabenazine (11C-DTBZ), a radioligand that binds to the presynaptic vesicular monoamine transporter. The 22q11.2DS group without Parkinson's disease demonstrated significant motor and olfactory deficits relative to controls. Eight (61.5%) were clinically classified with parkinsonism. Transcranial sonography showed a significantly larger mean area of substantia nigra echogenicity in the 22q11.2DS risk group compared with controls (P = 0.03). The 22q11.2DS patient with Parkinson's disease showed the expected pattern of severely reduced striatal 11C-DTBZ binding. The 22q11.2DS group without Parkinson's disease however showed significantly elevated striatal 11C-DTBZ binding relative to controls (∼33%; P < 0.01). Results were similar within the 22q11.2DS group for those with (n = 7) and without (n = 6) psychotic illness. These findings suggest that manifestations of parkinsonism and/or evolution to Parkinson's disease in this genetic at-risk population may include a hyperdopaminergic mechanism. Adequately powered longitudinal studies and animal models are needed to evaluate the relevance of the observed clinical and imaging phenotypes to Parkinson's disease and other disorders that are more prevalent in 22q11.2DS, such as schizophrenia.

Are rodent models of Parkinson's disease behaving as they should?

In recent years our understanding of Parkinson's disease has expanded both in terms of pathological hallmarks as well as relevant genetic influences. In parallel with the aetiological discoveries a multitude of PD animal models have been established. The vast majority of these are rodent models based on environmental, genetic and mechanistic insight. A major challenge in many of these models is their ability to only recapitulate some of the complex disease features seen in humans. Although symptom alleviation and clinical signs are of utmost importance in therapeutic research many of these models lack comprehensive behavioural testing. While non-motor symptoms become increasingly important as early diagnostic markers in PD, they are poorly characterized in rodents. In this review we look at well-established and more recent animal models of PD in terms of behavioural characterization and discuss how they can best contribute to progression in Parkinson's research.

Initial elevations in glutamate and dopamine neurotransmission decline with age, as does exploratory behavior, in LRRK2 G2019S knock-in mice

LRRK2 mutations produce end-stage Parkinson’s disease (PD) with reduced nigrostriatal dopamine, whereas, asymptomatic carriers have increased dopamine turnover and altered brain connectivity. LRRK2 pathophysiology remains unclear, but reduced dopamine and mitochondrial abnormalities occur in aged G2019S mutant knock-in (GKI) mice. Conversely, cultured GKI neurons exhibit increased synaptic transmission. We assessed behavior and synaptic glutamate and dopamine function across a range of ages. Young GKI mice exhibit more vertical exploration, elevated glutamate and dopamine transmission, and aberrant D2-receptor responses. These phenomena decline with age, but are stable in littermates. In young GKI mice, dopamine transients are slower, independent of dopamine transporter (DAT), increasing the lifetime of extracellular dopamine. Slowing of dopamine transients is observed with age in littermates, suggesting premature ageing of dopamine synapses in GKI mice. Thus, GKI mice exhibit early, but declining, synaptic and behavioral phenotypes, making them amenable to investigation of early pathophysiological, and later parkinsonian-like, alterations. This model will prove valuable in efforts to develop neuroprotection for PD.

Parkinson disease

Parkinson disease is the second-most common neurodegenerative disorder that affects 2-3% of the population ≥65 years of age. Neuronal loss in the substantia nigra, which causes striatal dopamine deficiency, and intracellular inclusions containing aggregates of α-synuclein are the neuropathological hallmarks of Parkinson disease. Multiple other cell types throughout the central and peripheral autonomic nervous system are also involved, probably from early disease onwards. Although clinical diagnosis relies on the presence of bradykinesia and other cardinal motor features, Parkinson disease is associated with many non-motor symptoms that add to overall disability. The underlying molecular pathogenesis involves multiple pathways and mechanisms: α-synuclein proteostasis, mitochondrial function, oxidative stress, calcium homeostasis, axonal transport and neuroinflammation. Recent research into diagnostic biomarkers has taken advantage of neuroimaging in which several modalities, including PET, single-photon emission CT (SPECT) and novel MRI techniques, have been shown to aid early and differential diagnosis. Treatment of Parkinson disease is anchored on pharmacological substitution of striatal dopamine, in addition to non-dopaminergic approaches to address both motor and non-motor symptoms and deep brain stimulation for those developing intractable L-DOPA-related motor complications. Experimental therapies have tried to restore striatal dopamine by gene-based and cell-based approaches, and most recently, aggregation and cellular transport of α-synuclein have become therapeutic targets. One of the greatest current challenges is to identify markers for prodromal disease stages, which would allow novel disease-modifying therapies to be started earlier.

Serotonin and dopamine transporter PET changes in the premotor phase of LRRK2 parkinsonism: cross-sectional studies

Background

Patients with Parkinson’s Disease (PD) may exhibit premotor neurochemical changes in dopaminergic (DA) and nondopaminergic systems. Using positron emission tomography (PET), we studied participants with leucine-rich repeat kinase 2 (LRRK2) mutations and with sporadic PD to assess whether DA and serotonin transporter (SERT) changes were similar in LRRK2 PD and sporadic PD, and whether asymptomatic LRRK2 mutation carriers exhibited PET changes in the absence of motor symptoms.

Methods

Between July 1999 and May 2016, we did two cross sectional PET studies at the Pacific Parkinson’s Research Centre (Vancouver, Canada) with LRRK2 mutation carriers with or without manifest PD, patients with sporadic PD, and age-matched healthy controls, all aged 18 years or older. Patients with PD were diagnosed by a neurologist with movement disorder training in accordance with the UK Parkinson’s Disease Society Brain Bank criteria. LRRK2 carrier status was confirmed by bi-directional Sanger sequencing. First, affected and unaffected LRRK2 carriers seen from July 1999 to January 2012 were imaged with PET tracers for the membrane dopamine transporter (DAT) and dopamine synthesis and storage (¹⁸F-6-fluoro-L-dopa; FDOPA) and compared with sporadic PD and age-matched healthy controls. Second, distinct groups of LRRK2 mutation carriers, sporadic PD patients, and age-matched healthy controls seen from November 2012 to May 2016 were studied with tracers for the SERT and vesicular monoamine transporter 2 (VMAT2). Striatal DAT binding, DTBZ binding, FDOPA uptake and SERT binding in multiple brain regions were compared using analysis of covariance adjusted for age.

Findings

Using data from 40 LRRK2 mutation carriers, 63 patients with sporadic PD, and 35 controls, we identified significant group differences in striatal DAT binding (all age ranges p<0·0001 in caudate and putamen) and FDOPA uptake (age 50 or lower in caudate, p=0·0002; all other age ranges p<0·0001; in putamen, all age ranges p<0·0001). Affected LRRK2 mutation carriers (n=15) had reduced striatal DAT binding and FDOPA uptake, comparable to sporadic PD of similar duration. Unaffected carriers (n=25) had greater FDOPA uptake and DAT binding than sporadic PD (n=63), with FDOPA uptake comparable to and DAT binding lower than healthy controls. Unaffected LRRK2 carriers (n=9) had significantly elevated SERT binding in hypothalamus (greater than healthy controls, 7 LRRK2 PD and 13 sporadic PD subjects; p<0·0001), striatum (greater than sporadic PD; p=0·02) and brainstem (greater than affected LRRK2 carriers; p=0·01) after adjustment for age. SERT binding in cortex was not significantly different between groups after age adjustment. Striatal DTBZ binding was reduced in all affected patients and asymmetrically reduced in one unaffected carrier.

Interpretation

Dopaminergic and serotonergic changes progress in a similar fashion in LRRK2 PD and sporadic PD, but unaffected LRRK2 mutation carriers exhibit increased SERT binding in striatum, brainstem and hypothalamus, possibly reflecting compensatory changes in serotonergic innervation preceding the motor onset of PD.

Funding

Canada Research Chairs, Michael J. Fox Foundation, National Institutes of Health, Pacific Alzheimer Research Foundation, Pacific Parkinson’s Research Institute, National Research Council of Canada

Changes in neural circuitry associated with depression at pre-clinical, pre-motor and early motor phases of Parkinson's disease

Although Parkinson's Disease (PD) is mostly considered a motor disorder, it can present at early stages as a non-motor pathology. Among the non-motor clinical manifestations, depression shows a high prevalence and can be one of the first clinical signs to appear, even a decade before the onset of motor symptoms. Here, we review the evidence of early dysfunction in neural circuitry associated with depression in the context of PD, focusing on pre-clinical, pre-motor and early motor phases of the disease. In the pre-clinical phase, structural and functional changes in the substantia nigra, basal ganglia and limbic structures are already observed. Some of these changes are linked to motor compensation mechanisms while others correspond to pathological processes common to PD and depression and thus could underlie the appearance of depressive symptoms during the pre-motor phase. Studies of the early motor phase (less than five years post diagnosis) reveal an association between the extent of damage in different monoaminergic systems and the appearance of emotional disorders. We propose that the limbic loop of the basal ganglia and the lateral habenula play key roles in the early genesis of depression in PD. Alterations in the neural circuitry linked with emotional control might be sensitive markers of the ongoing neurodegenerative process and thus may serve to facilitate an early diagnosis of this disease. To take advantage of this, we need to improve the clinical criteria and develop biomarkers to identify depression, which could be used to determine individuals at risk to develop PD.

Application of texture analysis to DAT SPECT imaging: Relationship to clinical assessments

Dopamine transporter (DAT) SPECT imaging is increasingly utilized for diagnostic purposes in suspected Parkinsonian syndromes. We performed a cross-sectional study to investigate whether assessment of texture in DAT SPECT radiotracer uptake enables enhanced correlations with severity of motor and cognitive symptoms in Parkinson's disease (PD), with the long-term goal of enabling clinical utility of DAT SPECT imaging, beyond standard diagnostic tasks, to tracking of progression in PD. Quantitative analysis in routine DAT SPECT imaging, if performed at all, has been restricted to assessment of mean regional uptake. We applied a framework wherein textural features were extracted from the images. Notably, the framework did not require registration to a common template, and worked in the subject-native space. Image analysis included registration of SPECT images onto corresponding MRI images, automatic region-of-interest (ROI) extraction on the MRI images, followed by computation of Haralick texture features. We analyzed 141 subjects from the Parkinson's Progressive Marker Initiative (PPMI) database, including 85 PD and 56 healthy controls (HC) (baseline scans with accompanying 3 T MRI images). We performed univariate and multivariate regression analyses between the quantitative metrics and different clinical measures, namely (i) the UPDRS (part III - motor) score, disease duration as measured from (ii) time of diagnosis (DD-diag.) and (iii) time of appearance of symptoms (DD-sympt.), as well as (iv) the Montreal Cognitive Assessment (MoCA) score. For conventional mean uptake analysis in the putamen, we showed significant correlations with clinical measures only when both HC and PD were included (Pearson correlation r = − 0.74, p-value < 0.001). However, this was not significant when applied to PD subjects only (r = − 0.19, p-value = 0.084), and no such correlations were observed in the caudate. By contrast, for the PD subjects, significant correlations were observed in the caudate when including texture metrics, with (i) UPDRS (p-values < 0.01), (ii) DD-diag. (p-values < 0.001), (iii) DD-sympt (p-values < 0.05), and (iv) MoCA (p-values < 0.01), while no correlations were observed for conventional analysis (p-values = 0.94, 0.34, 0.88 and 0.96, respectively). Our results demonstrated the ability to capture valuable information using advanced texture metrics from striatal DAT SPECT, enabling significant correlations of striatal DAT binding with clinical, motor and cognitive outcomes, and suggesting that textural features hold potential as biomarkers of PD severity and progression.

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Aim to enable image-based tracking of progression in Parkinson's disease

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Texture analysis of clinical dopamine transporter (DAT) SPECT images (DaTscans)

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Significant correlations with clinical, motor and cognitive outcomes